The need for improved methods and apparatus in digital communication has resulted from, among other things, (1) demands for data transmission of every form, from computer data banks to mobile radio communication, with ever increasing accuracy requirements and (2) the proliferation of data communication networks which must simultaneously serve many different users with a variety of rates and requirements.
Much attention has been given to the theoretical and practical solutions to multiple access communication in which communication is required over a shared channel by many users. Such systems include Time Division Multiple Accessing (TDMA) [1], Frequency Division Multiple Accessing (FDMA) [1], the Aloha system [2], the Capetanakis tree algorithm [3] (as modified by Gallager [4]) and Code Division Multiple Accessing (CDMA) [5]. The variety of systems can be attributed to the different requirements and resources of the communication system.
In the Aloha system, each user generates "packets" of data. These packets are transmitted over a communication channel as each are generated. If the transmission results in a collision with packets from other users, the user retransmits the packet after a random delay. Repeated transmission occurs until the packet is eventually successfully transmitted. This simple technique, by itself, is unstable since the channel soon becomes flooded with retransmissions. Accordingly, some form of stability control circuitry is required with the Aloha system.
Some of the proposed multiple access systems, such as the Aloha and Capetanakis tree, require feedback of information from the users to achieve transmitter synchronism. This feedback can be difficult to achieve in practice. Others, such as TDMA or FDMA require timing or frequency scheduling of transmission. For example, in the TDMA system, access to a channel is achieved by dividing time into frames. Each frame consists of M slots, one for each M user. Each M user transmits its packet in its assigned slot. Therefore, in TDMA, slot and frame synchronization is required.
Likewise, FDMA assigns frequency slots to the user. Such scheduling becomes inefficient when the number of users are large and each transmit infrequently.
A need exists, therefore, for a multiple access system, especially in mobile radio applications, in which feedback and scheduling is not required.
Code division multiple accessing (CDMA) systems have been described in which feedback and scheduling are not required. CDMA systems employ redundant coding before the message is transmitted. The users transmit without scheduling among themselves, thereby interfering with each other during transmission. The redundant coding helps the receiver to recover the transmitted information, in spite of this mutual interference.
In CDMA communication systems, it is assumed that there are a relatively large number of users and only a small fraction of users are transmitting at a given time. Such is typically the case in mobile radio communication systems.
A typical CDMA system is the so-called OR channel [6]. In the OR channel CDMA, each user signals by sending pulses and information is conveyed by the time position of the pulses. Pulses, due to other users, constitute interference. An idle user sends no pulse at all.
Spread spectrum CDMA [7] has been suggested for mobile radio communications. This scheme spreads the signal of each user over a spectral bandwidth which is substantially larger than that required for a single user. However, the throughput of this system is low and attenuation, due to distance, makes uniform reliable communication difficult to achieve for all users.
Furthermore, code-word synchronization at the receiver can be difficult to achieve in both spectrum CDMA and OR channel CDMA.
Accordingly, a need exists for a multiple access communication system without feedback, wherein it is unnecessary to assign different codes to different users, and which has relatively high throughput, and provides uniformly reliable communication among remote, as well as proximate, users and wherein synchronization of receivers is simple to achieve.